Many golf players desire to carry a golf ball longer distance more surely when they play golf. A golf club from which a long flight distance is expected is mainly a wood-type club (driver, fairway wood and utility), and the club head of the wood-type club is made of materials such as wood, stainless steel, aluminum alloy, titanium alloy and fiber reinforced plastic. Aside from advanced players such as professional golfers and top class amateur players, it is almost impossible for many of general golfers to hit a ball at the center of a face, which achieves a longest flight distance (hereinafter referred to as “sweet spot”), every time. Therefore, a principal target of golf club development in recent years is how to widen a right hitting portion which does not decrease the ball flight distance (hereinafter referred to as “sweet area”).
As a method of achieving the principal target, Document 1, for example, discloses a technology to increase the volume of a head for increasing the moment of inertia of the head and to provide a weight in the rear portion of the head for positioning the center of mass thereof rearward, that is, positioning the center of mass away from the face as much as possible when the club head is viewed from above, so as to achieve a small moment of rotation about the center of mass when a ball is hit at a spot out of the sweet spot (hereinafter referred to as “at the time of off-center shots”). Alternatively, Document 2 discloses a configuration in which a high repulsion area on a face portion is increased by combining different metallic materials in a crown, and Document 3 discloses a configuration in which a weight is provided on the inner surface of a crown to reduce the amount of back spin for the purpose of increasing flight distance, and Document 4 discloses a configuration in which the range of the repulsion area of the face portion is adjusted by increasing the rigidity of the crown to a level higher than that of the sole. As described in these documents, various modifications are applied to a crown or, in contrast, various modifications are applied to a sole in publicly known techniques. In addition, Document 5 discloses a configuration in which the coefficient of repulsion is increased by specifying the range of mechanical impedance of a golf club head with respect to the frequency area of the mechanical impedance of a golf ball, this is also known in public.    Document 1: JP-A-10-263122    Document 2: JP-A-2005-348895    Document 3: JP-A-2004-275751    Document 4: JP-A-2006-461    Document 5: JP-A-8-224328
However, the inventions disclosed in Documents 1 to 5 do not necessarily satisfy golfers under the present situation, and there still exists a problem that the flight distance cannot be dramatically improved. Since modification in shape of the golf club head causes various problems in terms of rules, it is desired to develop the golf club which is able to improve the flight distance at a normal shot and further satisfies the golfers without significantly changing the current shape.